The Red was created by writer Jamie Delano for his first Animal Man story arc, Flesh and Blood, which involved the death of Buddy Baker and his subsequent travel through the Red to regain his original body.
As featured in Jeff Lemire's Animal Man, The Red also exists on a physical plane as well to some extant wherein the will of the current avatar of the Red is able to shape the landscape.
The Red connects all living things through the Life Web and exists in a balance between the Green and the Rot. As the current avatar of the Red, Maxine Baker, is too young to take her mantel and Alec Holland is reluctant to take his role as the avatar of the Green, the Rot has grown in power. Parts of the Rot are appearing within the Red, containing small Rotling creatures.
Buddy Baker and his daughter Maxine are actually able to travel to the Red through the roots of a tree floating in a desert. Buddy enters again on his own after his body is taken over by the Rot.
Avatars of the Red
Each time either the Red, the Green or the Rot overstep their place and the balance of life is disrupted, an Avatar is raised by each element as a champion to defend that group's interests and restore balance. There have been a great number of such Avatars throughout the DC's timeline such as Jacob Mullin, Buddy Baker and eventually his daughter Maxine who is destined to become "Animal Woman".